Supercritical fluids possess several characteristics which make them interesting for electrodeposition. Supercritical fluids have low viscosity, allowing fast mass transport and therefore rapid deposition. Additionally, adjusting the pressure and temperature influences the density and viscosity of the solvent, therefore allowing tuning of the solvent properties^{1 2}. Low surface tension and phase separation also allows penetration of electrolyte into porous templates, meaning high aspect ratio nanowires can be deposited, with reported diameters as low as 3 nm³. This is a capability pertinent to the manufacture of the next generation of p-block based electronic devices, such as thermoelectric or phase change memory materials. As the current fabrication methods begin to reach their limits of size and control capabilities, extracting the maximum potential from these materials requires a new approach.

Where possible, aqueous conditions remain the most popular for electrodeposition, however its small cathodic limit can put refractory metals out of reach. Supercritical difluoromethane (scR32) is a promising solvent for supercritical electrodeposition. It has a large cathodic limit (-2 V), and a high enough dielectric constant to provide reasonable conductivity, and coordinates poorly with reagents¹. Electrodeposition from scR32 has yielded films and nanowires of several p-block metals, including Te, Bi, Sb and Ge^4-6. Extending this capability to alloys and compounds is the next challenge.

We present an introduction into the practical aspects of electrochemistry in supercritical fluids, often a non-trivial task. We will also present preliminary results concerning electrodeposition from a bismuth and tellurium electrolyte. The films produced were of varying, controlled composition. Bismuth telluride is a thermoelectric material, with the highest known room temperature thermoelectric figure of merit⁷, and is currently the subject of significant research efforts. The combination of supercritical fluids and electrodeposition holds significant promise for the fabrication of nanostructured materials.

This work has been supported by the EPSRC Programme Grant, Advanced Devices by Electroplating (EP/N0354371/1). Work from the EPSRC Programme Grant EP/I013394/1 also contributed.

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